Glass body having a semicrystalline surface layer and method of making it



United tates Patent GLASS BODY HAVING A SEMICRYSTALLINE SURFACE LAYER AND METHOD OF MAK- ING IT Joseph S. Olcott, Painted Post, and Stanley D. Stookey, Corning, N.Y., assiguors to Corning Glass Works, Corning, N.Y., a corporation of New York No Drawing. Filed July 1, 1959, Ser. No. 824,178

3 Claims. (Cl. 49-77) Another object is to provide a particular range of glass compositions especially suited for use with such method.

Another object is to provide a glass body of high mechanical and thermal strength having a semicrystalline surface by such method.

For some purposes, such as an increase in the mechanical and thermal strength of a glass body, it is desirable to provide it with a compressive stress in and parallel with its surface. One method of accomplishing this is the process known as tempering, which does not alter the oxide composition of the glass and which comprises heating the glass body while avoiding devitrification thereof and then rapidly cooling it. The maximum mechanical strength or modulus of rupture of an abraded tempered glass article of the type used for culinary purposes does not normally exceed 12,000l6,000 p.s.i. as compared to an abraded annealed glass article, the modulus of which is approximately 6,0008,000 p.s.i.

A higher compressive stress may be produced in the surface of a glass body by the introduction therein of lithium ions by the ionic migration of lithium into the surface in exchange for the potassium and sodium ions of the glass. The surface layer of the glass body thereby acquires a different chemical composition and a lower expansion coefficient than the unaffected interior of the body, Which results in a compressive stress in its surface layer. If the glass contains suitable proportions of alumina and silica, such method may also result in the formation of beta-spodurnene crystals in its surface, which further lowers its expansion coefficient and increases the surface compressive stress. Such method requires the immersion of the glass body in' a bath containing a molten lithium salt heated above the strain point of the glass. Molten salt baths are diflicult and dangerous to manipulate.

We have now discovered a new method of producing similar useful effects whereby higher mechanical strengths can be obtained without the use of a molten salt bath while the chemical composition of the glass body remains unchanged throughout.

Broadly the new method comprises heat treating a glass body comprising by weight 65-72 parts of SiO not less than 4 parts of Li O, 22.5-30 parts of A1 0 the weight ratio Li O/A1 O being not more than about 0.3/1, the total SiO Li O and A1 0 being at least 100 parts by weight, and at least one crystallization catalyst in the indicated amount selected from the class consisting of 0.1-3.5 parts of TiO 014 parts of B 0 0.42 parts of Na O and 0.5- parts of PhD, the total composition being not more than 110 parts by weight, by heating it at a temperature at which its viscosity is between 10 and 10 poises for a time ranging from about 1 hour to about 40 hours respectively until microscopic crystals of beta-eucryptite are formed within its surface. The temperature corresponding to a viscosity of 10' poises is somewhat above the softening point, which by definition is the temperature at which the viscosity is about 10' poises and heat treatments at these temperatures tend to cause deformation unless the glass article is adequately supported. At viscosities less than 10' poises the deformation tendency is impracticably great. On the other hand, heat treatments at viscosities greater than 10 poises require times too long to be practicable.

In the above defined range of compositions, glasses having SiO contents at or near the minimum and Li O contents at or near the maximum have viscosities in the neighborhood of 10 poises at about 970 C. and glasses having SiO contents at or near the maximum and U 0 contents at or near the minimum have viscosities in the neighborhood of 10 poises at about 770 C.

As a result of the new method a thin, compressive, semi-crystalline layer containing a multiplicity of microscopic and submiscroscopic crystals of beta-eucryptite (Li O-Al O -2SiO as the primary crystalline phase and having a linear thermal expansion coefiicient substantially lower than that of the glass per se is formed in and below its surface.

It is characteristic of the method of this invention that it causes such crystallization to occur only in the surface of the glass while leaving the interior thereof substantially unchanged with its original transparency; and that the resulting article as a whole is substantially transparent on account of both the thinness of such semicrystalline layer (about 0.1 mm.) and the relative similarity of the refractive indices of the crystals and the glass, there being at the most a slight translucency of the surface. The linear expansion coefficient of the semicrystalline layer is substantially lower than that of the glass interior, because the crystalline beta-eucryptite, which is formed, has a cubical expansion coefficient in the neighborhood of zero. Moreover, the expansion coefficient of the residual glassy matrix surrounding such crystals is also relatively lower than that of the unchanged glass of the interior, because the formation of the beta-eucryptite has left the matrix with a lower available content of Li O per se, which has a high thermal expansion factor. The lower expansion coeflicient of the semicrystalline surface layer relative to that of the interior of the glass establishes a uniform compressive stress in and parallel to the surface after the article is cool whereby the'modulus or rupture of the article is substantially increased. The formation of such semicrystalline surface layer is dependent upon the presence in the glass of a nucleating agent or crystallization-promoting agent.

The proportions of the above-recited ranges of SiO Li O, A1 0 and crystallization catalyst are critical for the purpose of this invention and such limits should be maintained for the following reasons: When the SiO content is below 65 parts by weight and the A1 0 content is above 30 parts the crystallization catalyst is not essential for promoting the surface crystallization but when the SiO content is over 72 parts and the A1 0 content is less than 22.5 parts a satisfactory semicrystalline surface layer cannot be produced. The liquidus of the glass becomes too high and the glass crystallizes throughout, if

- the Li O content is more than about /3 the amount of tially more than about 0.3/1 tend to crystallize throughout when heat treated, More than about 3.5% TiO The modulus of rupture is measured by supporting incauses crystallization to occur throughout the glass; exdividual rods of the semicrystalline product, about cessive B or Na O or PbO causes a diminution in the inch in diameter and 4 inches long, on 2 knife edges mechanical strength of the finished product. The presence spaced 3 /2 inches apart and individually loading them on of any substantial amount of K 0 weakens the final 2 downwardly acting knife edges about /4 inch apart product. and centrally spaced from the lower knife edges until Other compatible metal oxides may also be present breakage of the rods occurs. To make the results more provided their total amount does not exceed about comparable, the rods are first abraded by being. rolled parts. Such compatible metal oxides include the oxides in a ball mill for minutes with grit silicon carbide. of the metals of the second periodic group. 10 Ordinarily, five or more rods are thus tested to obtain In another application Serial No. 824,179, filed July the average value which is calculated in p.s.i. Abraded 1, 1959 concurrently herewith, we have disclosed and rods of annealed glass in general, when measured in this claimed a method and an article similar to the method manner, show moduli of rupture ranging from 6,000 to and the article of this application but utilizing glasses 8,000 p.s.i. which contain less than 65 parts of SiO and more than 5 It is believed that mlCTOSCOPIC cracks in the surface of 30 parts of A1 0 and in which TiO B 0 Na O and the glass before it is heat treated, may have a weakening PbO are optional but not essential constituents. efiect on the modulus of rupture of the finished article.

The present invention is illustrated by way of examples In order to eliminate such cracks and avoid such effect in Table I in which are shown the compositions of glasses the glass article may, if desired, be acid washed before falling within the above-stated range calculated from 20 being heat treated. Acid washing for this purpose is their respective batches to the oxide basis in parts by known as fortification and preferably comprises immersweight, exclusive of minor impurities in the batch maing the article for about 10 seconds in a solution comterials. posed of equal parts of 70% aqueous HF, concentrated Table I The same compositions are shown in Table H in Weight H 80 and water; rinsing it in a 5% aqueous solution of percent together with the times (hrs) and temperatures HNO and thereafter rinsing it in water. Such acid C.) of heat treatment and the moduli of rupture washing may be repeated several times, if desired, al- (p.s.i. l0- of the resulting article having a semicrysthough a single treatment ordinarily is effective, talline surface layer. In carrying out the invention, articles composed of the Table II The batch materials for the above glasses may comprise above described glasses are heat treated at the viscosities any materials, either oxides or other compounds, which, and for the time set forth above or until their moduli on being fused together, are converted to the desired oxide of rupture exceed about 15,000 or, preferably, 20,000 compositions inthe desired proportions. Preferably, comp.s.i. It has been found desirable to limit the rate of pounds such as petalite or spodumene, comprising 2 or temperature increase of articles having a thickness of more of the oxides of the final composition should be about inch or more to a maximum of about 5 C. used in order to provide glasses of optimum homogeneity. per minute, in order to avoid excessive thermal gradients Although it is immaterial whether the batches contain which might cause shattering, although much higher rates oxidizing or reducing agents, they preferably should 50 can be tolerated if the bodies are thin and substantially either contain oxidizing agents or should be neutral and uniform in cross section. Instead of raising the temcontain neither an oxidizing nor a reducing agent in order perature and holding it for a specific length of time, the to avoid the difiiculties normally encountered in fining temperature may be raised continually through the stated reduced glasses. Fining of the present glasses was acrange of viscosities at a rate so slow that surface cryscomplished by adding As O to the batch, Too large an 5 tallizadon adequate for the present purpose will have amount of As O say more than 1% tends to diminish occurred when a viscosity in the neighborhood of 10" the strength of the final product. The reason for this is poises or less is attained.

not known. The As O was omitted from the tables for We have found that only glasses which comprise pri convenience, since the residual amount normally remainmarily of Li O, Al O and SiO within the above stated ing in the glass is too small to have any material effect ranges of proportions and having the stated amounts of on its fundamental properties. The batches should be crystallization catalysts incorporated therein, are effective melted for at least 4 hours or more at about 1400 C. for carrying out the new method and producing the above or as much higher as may be necessary to produce homodescribed new article.

geneous melts in crucibles, pots or tanks depending upon What is claimed is:

the Size Ofthe meli- 76, 1.- T e method of making a glass body with a high than 4 parts of Li O, 225-30 parts of A1 the Weight ratio Li O/A1 O being not more than about 0.3/1, the total SiO U 0 and A1 0 being at least 100 parts by weight, and at least one crystallization catalyst in the indicated amount selected from the class consisting of 0.1-3.5 parts of TiO 0.1-5 parts of B 0 0.4-2 parts of Na O and 05-10 parts of PbO, the total SiO Li O, A1 0 and crystallization catalyst being not more than 110 parts by weight by heating it at a temperature at which its viscosity is between and 10 poises for a time ranging from about 1 hour to about hours respectively until microscopic and submicroscopic crystals of beta-eucryptite are formed within its surface.

2. The method of claim 1 in which the glass body is fortified by acid washing before being heat treated.

3. An article comprising a glass body having in its entire surface a thin, compressive, semicrystalline layer containing a multiplicity of microscopic crystals of betaeucryptite and having a linear thermal expansion coefli cient substantially lower than that of the glass body, the oxide composition of the article being substantially the 25 Industry, page 307.

same throughout and comprising -72 parts of Si0 not less than 4 parts of L120, 22.5-30 parts of A1 0 the weight ratio Lip/A1 0 being not more than about 0.3 1, the total SiO Li o and A1 0 being at least parts by weight, and at least one crystallization catalyst in the indicated amount selected from the class consisting of 0.1-3.5 parts of TiO 0.1-5 parts of B 0 0.4-2 parts of Na;,0 and 05-10 parts of PhD, the total SiO Li,0, A1 0, and crystallization catalyst being not more than 10 parts by weight.

References Cited in the file of this patent UNITED STATES PATENTS 15 2,143,796 Phillips Ian. 10, 1939 2,241,511 Greene May 13, 1941 FOREIGN PATENTS 551,562 Canada Ian. 14, 1958 OTHER REFERENCES Article by Pavlushkin, in May 1958, issue of The Glass Industry, page 275.

Article by Knapp, in. June 1959, issue of The Glass 

1. THE METHOD OF MAKING A GLASS BODY WITH A HIGH MODULUS OF RUPTURE HAVING ON ITS SURFACE A THIN, COMPRESSIVE, SEMI-CRYSTALLINE LAYER OF THE SAME OXIDE COMPOSITION AS THE GLASS, WHICH COMPRISES HEAT TREATING A GLASS BODY COMPRISING BY WEIGHT 65-72 PARTS OF SIO2, NOT LESS THAN 4 PARTS OF LI2O, 22.5-30 PARTS OF AL2O3, THE WEIGHT RATIO LI2O/AL2O3 BEING NOT MORE THAN ABOUT 0.3/1, THE TOTAL SIO2, LI2O AND AL2O3 BEING AT LEAST 100 PARTS BY WEIGHT, AND AT LEAST ONE CRYSTALLIZATION CATALYST IN THE INDICATED AMOUNT SELECTED FROM THE CLASS CONSISTING OF 0.1-3.5 PARTS OF TIO2, 0.1-5 PARTS OF B2O3, 0.4-2 PARTS OF NA2O AND 0.5-10 PARTS OF PBO, THE TOTAL SIO2, LI2O, AL2O3 AND CRYSTALLIZATION CATALYST BEING NOT MORE THAN 110 PARTS BY WEIGHT BY HEATING IT AT A TEMPERATURE AT WHICH ITS VISCOSITY IS BETWEEN 10**7 AND 10**10 POISES FOR A TIME RANGING FROM ABOUT 1 HOUR TO ABOUT 40 HOURS RESPECTIVELY UNTIL MICROSCOPIC AND SUBMICROSCOPIC CRYSTALS OF BETA-EUCRYPTITE ARE FORMED WITHIN ITS SURFACE.
 3. AN ARTICLE COMPRISING A GLASS BODY HAVING IN ITS ENTIRE SURFACE A THIN, COMPRESSIVE, SEMICRYSTALLINE LAYER CONTAINING A MULTIPLICITY OF MICROSCOPIC CRYSTALS OF BETAEUCRYPTITE AND HAVING A LINEAR THERMAL EXPANSION COEFFICIENT SUBSTANTIALLY LOWER THAN THAT OF THE GLASS BODY, THE OXIDE COMPOSITION OF THE ARTICLE BEING SUBSTANTIALLY THE SAME THROUGHOUT AND COMPRISING 65-72 PARTS OF SIO2, NOT LESS THAN 4 PARTS OF LI2O, 22.5-30 PARTS OF AL2O3, THE WEIGHT RATIO LI2O/AL2O3 BEING NOT MORE THAN ABOUT 0.3/1, THE TOTAL SIO2, LI2O AND AL2O3 BEING AT LEAST 100 PARTS BY WEIGHT, AND AT LEAST ONE CRYSTALLIZATION CATALYST IN THE INDICATED AMOUNT SELECTED FROM THE CLASS CONSISTING OF 0.1-3.5 PARTS OF TIO2, 0.1-5 PARTS OF B2O3, 0.4-2 PARTS OF NA2O AND 0.5-10 PARTS OF PBO, THE TOTAL SIO2, LI2O, AL2O3, AND CRYSTALLIZATION CATALYST BEING NOT MORE THAN 110 PARTS BY WEIGHT. 